Messaging transport servers such as email servers typically receive, store, and forward messages to the subsequent server(s) in the communications pathway to the final destination. In such cases, multiple high availability approaches can be deployed to ensure that the messages exist on at least two transport servers in order to be resilient against hardware and/or software failures.
However, it is oftentimes the case where software updates are provided by the vendor and customers do not always update to the latest version and associated capabilities due to limited resources, for example. In other words, high availability for message transfer agents can be provided in a later version of the messaging server, yet earlier (or legacy) server versions lack the capability to interact with all of the high availability capabilities of the updated server versions.
When receiving a message submission from a client or server over standard protocols (e.g., SMTP-simple message transfer protocol), the transport is supposed to ensure that delivery will be completed before the receiving server accepts responsibility for the message. This generally applies to any inbound sessions over which messages are received from anonymous servers. Hence, customers need to be provided a way to use legacy systems with updated versions of messaging entities in order to provide resilient message redundancy.